Connecting Modbus devices to Didactum fault reporting systems
Modbus is a serial communication protocol. Modbus has long since become a de facto standard communication protocol. Modbus enables communication between many devices connected to the same network, for example, a system that measures temperature and humidity and transmits the results to a control and monitoring unit.
Modbus protocols
As of February 2019, Didactum's Ethernet-based measurement and control systems supported the following Modbus protocols:
Modbus RTU – This is used in serial communication and employs a compact, binary representation of data for communication. The RTU format follows commands/data with a cyclic redundancy checksum as a verification mechanism to ensure data reliability. Modbus RTU remains the most common Modbus implementation available.
Modbus communication
Every device intended to communicate via Modbus receives a unique address. In serial networks, only the node designated as the master may execute a command. Within an Ethernet network, any device may execute a Modbus command, although typically only the master device does so (Modbus TCP/IP will be supported by Didactum measurement and control devices in the future).
A Modbus command contains the Modbus address of the device for which it is intended (1 to 247). Only the addressed Modbus device responds to the command. An exception to this are certain commands sent to node 0, which are executed but not explicitly acknowledged. Modbus commands contain a checksum to detect transmission errors. Basic Modbus commands can instruct an RTU to change the value in a register, control an I/O port, or read data. The values contained in the registers can also be transmitted by the Modbus devices.
RS-485 interface
RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems.
RS-485, also known as EIA-485, is an industry standard that defines the electrical characteristics of transmitters and receivers in serial communication systems. Electrical signaling is balanced, and multi-node connections are supported. The standard was jointly adopted by the Telecommunications Industry Association and the Electronic Industries Alliance (TIA/EIA). Digital communication networks can be used effectively over long distances and even in environments with significant electromagnetic interference. Multiple receivers can be connected to such a network via a linear multi-drop bus.
These characteristics make RS-485 suitable for use in industrial control systems and related applications.
Connecting Modbus devices to Ethernet-based measurement and control systems
The total cable length should not exceed 1,200 meters. The last devices in this chain must be terminated with a 120-ohm resistor. Please ensure that both resistors are correctly connected to the terminals. The MODBUS specification requires the use of terminating resistors (120 to 150 ohms, 0.25 W) at both ends.
Please also note that RS485 or Modbus does NOT support star wiring or spur connections!
All Modbus devices must be connected serially (in a chain). The SNMP-based Didactum monitoring device can be installed at the beginning or in the middle of the Modbus chain!
Branches from the main Modbus line are also possible. However, Didactum experts advise against this type of Modbus cabling, as it can often lead to problems. Ensure that each branch is as short as possible.
Connecting Modbus RTU devices to the IP-based control unit
Please note that connecting Modbus devices to the IP-based monitoring systems 50 / 100 (DC) / 400 / 500 (DC) requires the Modbus board (available at an additional cost). The premium monitoring systems of the 600 (DC) and 700 (DC) series can be equipped with an RS485 to USB adapter (optional). Modbus RTU devices are then connected to this adapter.